Okay, you've made some changes locally, saved them as commits using git commit, and now you want to share these updates with others or simply back them up on your remote repository (like GitHub or GitLab). This is where the git push command comes into play. It's the primary way you send your committed changes from your local repository to a remote repository.

Think of it like uploading a document you've saved on your computer to a shared online folder. Your series of commits are the updates to the document, and the remote repository is the shared folder where everyone can see the latest version.

How git push Works

The basic structure of the command is straightforward:

git push <remote-name> <branch-name>

Let's break this down:

git push: This is the command itself, telling Git you intend to send committed changes outward to a remote location.
<remote-name>: This specifies which remote repository you want to send the changes to. Remember from the previous sections how you might have added a remote using git remote add or how cloning automatically sets one up? Very often, the default remote you cloned from or added manually is named origin. So, you'll frequently use origin here. You can always list your configured remotes and their URLs using git remote -v to be sure.
<branch-name>: This specifies which local branch's commits you want to send. If you've been making commits directly on the main line of development, this is typically main (or sometimes master in older projects). If you created a feature branch to work on something specific (as discussed in the chapter on branching), you would use that branch's name here, for example, feature-login.

A Common Example

The most frequent use case is pushing commits you've made on your local main branch to the origin remote:

git push origin main

When you execute this command, Git performs a few steps:

It looks at the history of your local main branch.
It communicates with the origin remote repository to see the history of its main branch.
It determines which commits exist on your local main branch but are missing from the origin/main branch.
It transfers (pushes) those missing commits and associated data to the origin remote.
Finally, it updates the main branch pointer on the origin remote to point to the latest commit you just pushed.

This diagram shows how the git push command takes commits from your local repository and sends them to update the remote repository.

Pushing a New Branch for the First Time

What if you've created a completely new branch locally, say fix-typos, made some commits on it, and now want to share it on the remote? The first time you push this new branch, the remote repository (origin) doesn't even know it exists yet. You need to tell Git explicitly to create this branch on the remote and, importantly, to link your local branch to this new remote branch.

You accomplish this using the -u option (which is shorthand for --set-upstream):

git push -u origin fix-typos

This command performs two significant actions:

It pushes your local fix-typos branch, along with all the commits reachable from it that the remote doesn't have, to the origin remote. This creates the fix-typos branch on the remote repository.
It establishes a "tracking relationship" between your local fix-typos branch and the origin/fix-typos branch on the remote.

This tracking relationship is convenient. Once it's set up (by using -u on the first push), for any subsequent pushes you make while on the fix-typos branch, you can often just type the simpler command:

git push

Git will automatically know that you intend to push the current branch (fix-typos) to its corresponding tracked remote branch (origin/fix-typos). This tracking relationship also simplifies pulling changes, which we will cover in the git pull section.

What if Pushing Fails? (Non-Fast-Forward Errors)

Occasionally, your attempt to push might be rejected by the remote repository. Git will usually display an error message containing terms like rejected, failed to push some refs, and often non-fast-forward.

This error typically signifies that the remote branch has received new commits since the last time you synchronized your local repository with it (using commands like git fetch or git pull). Imagine you started working based on commit B, and you added commit C. Meanwhile, someone else also started from commit B, added commit D, and successfully pushed D to the remote before you tried to push C.

This situation occurs when the remote branch has moved forward with new commits since you last updated your local copy. Pushing your changes would overwrite the existing, different history on the remote.

Git prevents this "non-fast-forward" push because accepting your commit C would cause the remote repository to lose commit D. The histories have diverged.

The standard solution is to first incorporate the remote changes into your local branch before pushing again. You need to fetch the latest changes (including commit D) and merge them with your local work (commit C). The git pull command is designed for this fetch-and-merge operation, and we will examine it in detail in its own section. For now, the main takeaway is that a "non-fast-forward" error usually means you need to pull before you can push.

Authentication Considerations

Pushing changes involves writing data to the remote repository, so it almost always requires authentication. You need to prove to the remote server (like GitHub, GitLab, or Bitbucket) that you have the necessary permissions to modify the project.

The authentication method depends on the protocol you are using to connect to the remote (usually HTTPS or SSH):

HTTPS: You might be prompted for your username and password. More commonly nowadays, hosting platforms require you to use a Personal Access Token (PAT) instead of your regular password for security reasons. You generate this token on the platform's website and use it like a password when Git prompts you.
SSH: This method uses cryptographic key pairs. You add your public SSH key to your account on the hosting platform, and Git uses your private key (stored securely on your computer) to authenticate automatically without needing passwords or tokens for each push.

If git push fails with an authentication error, Git's message will usually point you in the right direction. You may need to check your stored credentials, generate a PAT, or ensure your SSH keys are correctly configured and added to the remote platform. Consult the documentation for your specific Git hosting service for detailed instructions.

Using git push effectively allows you to share your contributions, collaborate with team members, and maintain backups of your project history on a remote server. It's a fundamental command for working with Git beyond your local machine.